When selecting a diode, many people tend to focus on parameters like the forward voltage drop and current limit values, often overlooking an equally critical aspect: ensuring that the diode's actual power dissipation doesn't exceed its rated PCM value during operation. Exceeding this limit could result in the diode being overloaded and potentially damaged. Understanding the PCM value is therefore crucial for any engineer working with semiconductors.
Diodes, as fundamental electronic components, come in various types based on their semiconductor material—such as silicon and germanium—or their application—like rectifier, detection, switching, and Zener diodes. Regardless of classification, one key parameter that significantly impacts a diode’s operational safety is its power dissipation capability.
The term “power dissipation†refers to the maximum allowable power that a diode can handle without exceeding specified limits in its performance metrics. This value is calculated as the difference between the input power and the output power delivered by the diode. In linear modes, calculating power dissipation is straightforward, typically following formulas like PD=I²R or PD=U²/R. However, in switching states, the calculations become more complex due to dynamic factors.
The maximum power dissipation of a diode is closely tied to its junction temperature. Silicon diodes can generally tolerate a maximum junction temperature of up to 150°C, whereas germanium diodes have a lower limit of around 85°C. As the operating temperature rises, so does the junction temperature, eventually reaching the point where further power dissipation would cause damage. Additionally, the physical size of the diode plays a role in its power handling capacity. Larger packages usually mean better heat dissipation, which translates into higher power dissipation capabilities.
For instance, take the 1N4448HWS diode. According to its datasheet, it has a maximum power dissipation (PD) of 200mW at an ambient temperature of 25°C, assuming it is soldered onto an FR-4 PCB under standard conditions. However, as the ambient temperature increases, the maximum power dissipation decreases due to reduced thermal conductivity. At 75°C, the dissipated power might drop to 0.4W. Conversely, improving the heat dissipation environment can increase the allowable power dissipation. For example, adding a heatsink could allow the diode to handle up to 1.7W at the same ambient temperature.
Another important factor is thermal resistance, denoted as Rja (junction-to-ambient thermal resistance), measured in °C/W. Lower thermal resistance indicates better heat transfer efficiency, while higher values suggest poorer heat dissipation. Rja essentially quantifies the temperature difference across a 1W heat flow path.
Returning to the 1N4448HWS, its datasheet specifies a thermal resistance of 625°C/W. Using this data, we can calculate the relationship between ambient temperature and power dissipation:
At 0 to 25°C, the power dissipation remains constant at 200mW. Between 25°C and 150°C, the relationship becomes linear, with power dissipation dropping to zero at 150°C—the point at which the diode ceases to function. This information allows us to compute the thermal resistance as well:
\[ \text{Rja} = \frac{\Delta T}{P} = \frac{150 - 25}{0.2} = 625 \, \text{°C/W} \]
From this, we derive the equation:
\[ P_D = -\frac{1}{625}(T_A - 25) + 0.2 \quad \text{(for } T_A \geq 25 \text{)} \]
This formula helps determine the maximum allowable power dissipation at different ambient temperatures.
During the design phase, engineers prioritize maintaining safe operating temperatures. For example, if the ambient temperature is 25°C and the actual power is 100mW, the junction temperature will rise to 87.5°C, still within safe limits. However, at 200mW, the temperature could reach 150°C, which exceeds the safe operating range and poses a risk.
In summary, when working with diodes, both PD and Rja must be carefully considered. PD represents the maximum allowable power dissipation, which must never be exceeded. Rja reflects the diode’s thermal resistance, indicating its heat dissipation capability. Beyond current and voltage ratings, engineers must also account for power dissipation when designing circuits.
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[Attached Image: A diagram showing the relationship between temperature and power dissipation for the 1N4448HWS diode.]
Flexible Substrate
As the structural support and optical signal transmission pathway and medium, flexible substrates are playing ever-increasingly important roles in advanced optoelectronic display devices. The use of flexible substrates will significantly reduce the weight of flat panel displays and provide the ability to conform, bend or roll a display into any shape. Moreover, it will open up the possibility of fabricating displays by continuous roll processing, thus providing the basis for cost-effective mass production. Flexible substrate mainly used in thermoelectric refrigerator accessories, high-end car seats, car cold cup, car refrigerator, head display, car power, home appliances, medical devices, semiconductor chips, laser projection, optical device packaging in optical fiber communication and other fields.
Currently, there are mainly three types of candidates for flexible substrates: ultrathin glass, metal foil, and plastic (polymer) films. The raw material use for our flexible substrate is PE base double-sided copper clad with 0.3mm thickness. We are equipped with professional metal etching equipment and exposure development equipment. We use fine etching process and manufactures, we can guarantee that our etching flexible substrate can achieve double-sided etching of different graphics, alignment, neatly arranged, and no shedding, no incomplete, no pores, no inclusions and other appearance defects.
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